1. Field of the Invention
The present invention relates to an optical measurement apparatus. Specifically, the present invention relates to an optical measurement apparatus and an optical measurement method for performing optical tomographic observation.
2. Description of the Related Art
In recent years, attention has been paid to optical coherence tomography (OCT) that obtains an image reflecting a surface structure or an internal structure of an observation object by using light. Since the OCT is non-invasive to a human body, its application to, among others, medical fields and biological fields has been expected. In the ophthalmological field, an apparatus to form an image of a fundus or a cornea has been put in practical use. In the OCT, light from a light source is branched into signal light and reference light. The signal light is obtained by irradiating an observation object with the light from the light source. The reference light is not radiated to the observation object but is reflected by a reference light mirror. The signal light reflected from the observation object and the reference light are combined and interfere with each other, whereby a signal is obtained.
The OCT is roughly classified into time domain OCT and Fourier domain OCT according to a method for scanning a measurement position in an optical axis direction. In the time domain OCT, a low-coherence light source is used as the light source, and the scan in the optical axis direction is performed by scanning the reference light mirror at the time of measurement. As a result, only a component included in the signal light, whose light path length is coincident with that of the reference light, interferes, and a desired signal is demodulated by performing envelope detection on the obtained interference signal. On the other hand, the Fourier domain OCT is further classified into wavelength scanning OCT and spectral domain OCT. In the wavelength scanning OCT, a wavelength scanning light source capable of scanning a wavelength of emitted light is used. The scan in the optical axis direction is performed by scanning the wavelength at the time of measurement. A desired signal is demodulated by performing Fourier transform on wavelength dependence (interference spectrum) of detected interference light intensity. In the spectrum domain OCT, a wide-band light source is used as the light source. Generated interference light is spectrally dispersed by a spectroscope, and interference light intensity (interference spectrum) of each wavelength component is detected, which corresponds to performing the scan in the optical axis direction. A desired signal is demodulated by subjecting the obtained interference spectrum to Fourier transform.
In the OCT, as described above, image information reflecting the internal structure of the observation object can be obtained basically by the signal light and the reference light interfering with each other. “US 2014/0023255” discloses a technique to process a plurality of images obtained by changing a phase of signal light to increase an observable penetration depth. “JP 2011-196694 A” discloses a technique for the Fourier domain OCT to modulate a phase of reference light based on a wavelength of a light source. As a result, intensity of the reference light is substantially adjusted, whereby fine adjustment for a light path length is no longer required. “JP 2007-240453 A” discloses a technique for the time domain OCT to use a plurality of SLDs as well as to change a phase of reference light, thereby obtaining a difference in absorption distribution of an observation object caused by a difference in a wavelength. “WO 2001/42735” discloses a technique for the time domain OCT to use white light and an SLD as a light source. Specifically, the white light and the SLD are switched to each other, whereby resolving power in a depth direction is allowed to be varied. “US 2014/0204388” discloses a technique for the time domain OCT to physically scan an objective lens as well as to receive interference between signal light and interference light by four detectors with different phase conditions. As a result, it is no longer necessary to scan a mirror to adjust a phase of reference light.